Building-integrated photovoltaics are an attractive concept for economic generation of solar power. Integration of semi-transparent solar cells into windows is of particular interest, since it opens the prospect of employing the entire façade of a building for solar power generation, rather than simply employing the limited roof space. In order for such solar windows to be practical, costs must be low, and ideally they can be manufactured through existing coating methods employed in the glazing industry. They need to generate significant power, whilst still having good transparency. Furthermore, whilst coloured windows are interesting for novel applications, and a “splash” of colour may be desirable, the largest demand is for neutral-colour tinted windows with controllable levels of tinting.
Most recent approaches to achieve uniformly coated semi-transparency in solar cells have used organic solar cells or dye-sensitized cells (see, for instance, Kang, M. G., Park, N., Park, Y. J., Ryu, K. S. & H., C. S., Solar Energy Materials and Solar Cells, 75, 475-479, 2003). These technologies are solution-processable, representing a low-cost production method and easily scalable. However, their efficiencies are limited by fundamental charge transfer losses. As such, attempts to produce semi-transparent organic photovoltaics have resulted in either unsatisfactory performance or a lower visible transmittance than desired. To attain colour-neutrality, the active materials must be chosen carefully, often at a loss to overall efficiency (Amen, T. et al., Advanced Functional Materials, 20, 1592-1598, 2010). An option with thin-film solar technologies is to simply reduce the thickness of the absorber to allow transparency. Indeed, this is precisely what is done with amorphous silicon, currently being installed in BIPV. However, if this is done with any conventional semiconductor, such as amorphous silicon, the film will take on a red or brown tint due to the absorption coefficient increasing from the band-gap.
Semiconducting perovskites have recently emerged as a new and interesting class of photovoltaic materials. They offer solution-processable bulk semiconductors which can be fabricated using inexpensive and abundant materials. Single junction power conversion efficiencies of up to 12.3% have now been demonstrated (see, for instance, Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J., Science, 338, 643-7, 2012; Ball, J. M., Lee, M. M., Hey, A. & Snaith, H. J., Energy & Environmental Science, 2013, doi:10.1039/b000000x). The inventors' recent work has shown that high-efficiency perovskite solar cells can be produced at low temperatures and in a fully planar heterojunction thin-film architecture, greatly reducing fabrication costs and simplifying the design. These characteristics would make such perovskites ideal materials for fabrication of semi-transparent windows. However, although the methylammonium lead halide perovskite responsible for the highest efficiencies has absorption across the whole visible spectrum, uniform films that are thin enough to be semi-transparent have greater absorption in the high-energy end of the spectrum, as is observed with conventional semiconductors, meaning that they appear brown, or red in transmission. Though “bronze” is a choice for solar glazing, it does not represent the most desirable glazing option.